Probably everyone knows, but...
Pistol and stock attachment lugs were ground in the mistaken belief that a pistol with a lug was illegal or that a stock with an attaching "iron" was illegal. Neither was nor is.
But, possession of both a pistol with a lug and a compatible stock with an intact "iron" IS illegal unless the combination is registered as a short barrel rifle under the National Firearms Act.
There are exceptions; almost all C96 Mausers with ORIGINAL stocks have been removed from the purview of the NFA, as have some Lugers. The list is at:
http://www.atf.treas.gov/firearms/curios/sec3.htm
Note that this should NOT be confused with designation as a "curio and relic", either specifically or under the "50-year" rule. The "C&R" designation does not change the status of a firearm under the NFA. For example, most Thompson submachineguns are now in the C&R category, but they are still NFA firearms and NFA rules apply to possession and transfer.
Jim